Drop/replace `array_decode_utf8` from `benchmark_vlen.ipynb`

[jakirkham]

In PR ( dask/fastparquet#580 ) included in fastparquet release 0.6.0, array_decode_utf8 was dropped from fastparquet.speedups. We should either drop or replace this benchmark from notebook/benchmark_vlen.ipynb.

For context this notebook was originally added in PR ( #56 ).

[jakirkham]

cc @martindurant (in case you have any thoughts here; no worries if not 🙂)

[martindurant]

I had no idea that this existed!

[jakirkham]

The function or the benchmark? In either case think it was totally reasonable to drop that code from fastparquet (it looked kind of like an internal function). Just didn't know if there was something else we should be using here or if we should just drop it.

[martindurant]

The encode part was rolled into pack_byte_array, so note reachable from outside, definitely an internal thing.